Structured optical films are used in optical display systems and in other applications where control over the direction of light, transmitted and/or reflected, is desired to increase brightness, reduce glare, etc. Structured optical films are described generally in U.S. Pat. No. 4,906,070 (Cobb). Essentially, they comprise films of light transmissible materials in which a series of prisms are located such that the films can be used to redirect light through reflection and refraction. When used in an optical display such as that found in laptop computers, watches, etc., the structured optical film can increase brightness of an optical display by limiting light escaping from the display to within a pair of planes disposed at desired angles from a normal axis running through the optical display. As a result, light that would exit the display outside of the allowable range is reflected back into the display where a portion of it can be "recycled" and returned back to the structured film at an angle that allows it to escape from the display. That recycling is useful because it can reduce power consumption needed to provide a display with a desired level of brightness.
An undesirable effect of using a structured optical film in an optical display is the appearance of reflected moire caused by the interference of two periodic patterns. Moire effects are discussed in O. Bryngdahl, "Moire: Formation and Interpretation," Optica Acta, Vol. 24(1), pp. 1-13 (1977). In an optical display incorporating a single layer of structured optical film, the periodic patterns causing moire are the pattern in the film itself and the reflected image of the film pattern (as reflected by other surfaces in the optical display).
Some optical displays incorporate a second structured optical film in which the prisms are oriented at an angle with the prisms in the first optical film. That angle can be anywhere from greater than zero to 90.degree., although it is typically about 90.degree.. Although using two structured optical films can increase the brightness of the display within a narrowed viewing range, it can increase the effects of moire by providing a second plano surface (on the lower structured film) that reflects more light back through the periodic pattern in the first, or upper, structured film.
In addition, the second structured optical film may also lead to optical coupling that may result in uneven light transmission from the display, i.e., visible bright spots, streaks, and/or lines in the display. Optical coupling is caused by contacting, or very nearly contacting, a plano surface with the structured surface of a structured optical film.